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In this paper, for a 40MVA/110kV transformer, we explore the method of transformer refurbishment to 50MVA by using new technology and materials according to the current insulation structure, while keeping some imported parts, in order to achieve the purpose of extending transformer life, improving short-circuit resistance and saving money.
Along with the rapid growth of modern economy, the contradiction between the increase of load demand and the lack of original transmission and substation capacity is becoming more and more prominent, and the new equipment is constrained by factors such as capital, land resources and construction duration, so how to refurbishment of the used transformer, saving money while extending the life of the equipment has become a hot topic in the power industry.
As early as 2004, some scholars proposed that transformer refurbishment needs to be combined with increased short-circuit resistance, and the need to pay attention to the impact of expansion on group accessories. Some scholars have also modified a transformer for a power plant from 60MVA to 70MVA, while others have proposed to modify only the external cooling equipment of the transformer to make the transformer capacity higher.
With regard to transformer refurbishment, the power industry has done a lot of research, proposed a variety of solutions and gained practical experience.
However, these measures are not standardized and there is no mention of how to extend the life of the transformer after retrofitting.
A transformer station purchased a batch of 40MVA/110kV power transformers in the 1990s, which has been in operation for more than 20 years and in good condition, but with the rapid economic development of the region, electricity consumption has been increasing year by year, and the transformers have been operating under overload for more time, so the transformer life is at the end point.
Refurbished transformers can be purchased new, or old reconditioned transformers.
In recent years, the price of transformers has been rising, both because materials and labor costs have become expensive and because new transformers have become more expensive due to measures taken to meet the new short-circuit resistance requirements of the national grid.
refurbished transformers face increased costs, in addition, the main materials of early old transformers are generally imported materials, such as silicon steel sheets, bushings, switches, etc. These parts are still in good condition after 20 years of operation, reuse these imported parts to postpone the renewal of transmission and substation equipment, which can create considerable economic value, which has also become an This has become an important issue for the grid.
The production process of transformers in China 20 years ago was backward and the strength of insulation materials was low, which led to the volume of old transformers being larger than that of transformers of the same size today, and the insulation structure of current transformers is more reasonable and the quality of materials and manufacturing level is better.
The old transformer is the model SZ-40000/110, double-winding structure of the step-down transformer, the requirements of the reconditioned transformers are as follows.
(1) expand the capacity to 50MVA (25% expansion), the transformer life after the transformation and the new production transformer life equivalent.
(2) Considering that many materials of the old transformer are imported and in good operating condition, the original core, bushings and switches must be retained, while other parts will depend on specific conditions.
3）transformer refurbishment must meet the latest short-circuit resistance requirements of the grid.
(4) After considering the scrap copper depreciation, the refurbishment cost is lower than the new purchase of 50MVA/110kV transformer.
Under the premise of retaining the overall core of the transformer, the design of the winding becomes very critical. The design must not only meet the original core size, but also take into account the increase in current and losses caused by the surge in winding temperature after the capacity increase.
Reconditioned transformers are designed to increase the cross-sectional area of the wire so that the winding current density does not increase by a factor of 25%, and to control the temperature rise and losses so that they do not increase significantly.
Older transformers with larger insulation distances can increase the effective occupancy of copper in the same window volume. After the increase of copper, the electric density of low-voltage winding is only increased by 12% compared with the original design, and the electric density of high-voltage winding is increased by 9.7%.
In order to fill more copper wires inside the same window, the transformer refurbishment scheme reduces the winding spacing and the distance from the winding to the yoke compared with the old transformer, and also takes necessary insulation reinforcement measures to ensure the reliability of the insulation.
(1) The copper wrapping paper of high-voltage winding is increased from 4 layers to 5 layers, the role of wrapping is to eliminate the accumulation of fiber impurities in the oil in any case and the formation of small semiconductor bridges and the phenomenon of shorting the two electrodes, which can effectively increase the permitted field strength of the winding end.
(2) high pressure winding end increase a corner ring, its role is to separate the end of the large oil gap into a small oil gap, in order to increase the end of the oil pressure; the second is to increase the end of the discharge distance along the surface. The angle of the angle ring should be similar to the shape of the end equipotential line.
(3) the winding end of the line cake installed U-shaped pad, which can effectively increase the winding first end of the line cake along the surface of the creepage distance, increase the winding to ground discharge voltage, enhance the insulation strength of the transformer.
The simulation conditions are 480 kV for the high voltage applied lightning surge and grounding of the low voltage winding. The results show that after reducing the insulation distance, the field strength at the end of the high-voltage winding increases from 33.6kV/mm to 35.3kV/mm, but after using the above insulation reinforcement measures, the permissible value of field strength is 35.5kV/mm, which is a safety factor greater than 1 and meets the project requirements.
A lot of practical experience has been gained in the study of short-circuit resistance of transformers, and the main purpose of the study is to ensure the structural balance of the transformer, reduce the electrodynamic force during short-circuit, and strengthen the winding support. According to the special characteristics of this transformer refurbishment, on the basis of various experiences, the specific measures to improve the short-circuit resistance of this transformer are as follows.
(1) The winding wire adopts semi-rigid wire with yield strength more than 150MPa, and the wire is self-adhesive transposition wire to improve the amplitude stability.
(2) The winding is assembled with constant pressure drying treatment, increasing the number of bracing strips, and the span distance between adjacent bracing strips is less than 120mm.
(3) the use of hardness of the upper and lower pressure plate, axial preload force is greater than the calculated value of the axial short-circuit force, and leave enough margin.
4）Adopt high-density insulating paper punched into the pad, improve the elastic modulus of the pad, so as to improve the axial stability of the transformer. After taking the above measures, the short-circuit force calculation results of the modified transformer are shown in Table 2, and all the forces are guaranteed with sufficient margin.
Another important problem caused by the expansion is the temperature rise. A 25% increase in winding current results in a significant increase in load losses, and the resulting temperature rise is a key issue in transformer refurbishment.
After calculation, the maximum load loss of the transformer increased from 172.5kW before the transformation to 241.6kW. If no countermeasures are taken, the oil temperature and winding temperature will increase significantly, which is not conducive to prolonging the service life of the transformer.
Therefore, in addition to optimizing the internal cooling oil channels of the windings, it was necessary to add fans in the lower part of the transformer and change the cooling method of the transformer from ONAN to ONAF by installing three fans with an air volume of 480 m3/min at the bottom of the winding.
In order to completely solve the temperature rise problem and to ensure that the life of the refurbished transformers is more than 20 years, the retrofit solution uses high temperature insulating liquid natural ester insulating oil as the transformer insulation cooling medium, while the copper winding wrapping paper uses thermally modified paper, and the internal insulation system of the transformer must use high temperature insulation material of the liquid-immersed transformer specified semi-mixed insulation system. system, temperature rise allowable limits are in accordance with the semi-mixed insulation system requirements. The benefits of using natural ester insulating oil are.
(1) Natural ester insulating oil is extracted from plant seeds, which has a high natural degradation rate and reduces CO2 emission during the production and use process, responding to the call for green environment protection.
(2) The ignition point of natural ester insulating oil is higher than 300℃, which effectively reduces the probability of transformer fire and simplifies the fire-fighting measures in substations.
(3) The saturated water content of natural ester insulating oil is higher, which plays a role in drying the internal insulating materials during the operation of transformers and helps to extend the life of transformers.
(4) Natural ester insulating oil transformers have better overload capacity. Where the allowable temperature rise is selected in accordance with GB1094.14 standard for semi-mixed insulation system.
With the semi-mixed insulation system and natural ester insulating oil, the oil temperature and hot spot temperature rise of the winding have a considerable margin according to the permitted value of the standard, and the life of the transformer can be ensured to be comparable to the life of a newly purchased transformer.
Due to the 25% expansion, the load losses of the transformers increased by about 40%, the no-load losses remained the same and the weight of the transformers increased by 0.9t.
The transformers are still in good condition with imported components. Since some parts of the transformer are still in good condition, the cost of retrofitting is about 68.8% of that of purchasing a new transformer. The advantages of the modified transformer are as follows.
1) The quality is guaranteed by continuing to use the imported parts of the core, switch and bushing of the original transformer.
2) Modified to semi-mixed insulation system with natural ester insulating oil, longer life and higher overload capacity.
3) The transformer has sufficient short-circuit resistance.
4) The transformation price is more economical than purchasing a new brand-new transformer.
In this paper, I have increased the capacity of a 110kV transformer, which has been in operation for more than 20 years, from 40MVA to 50MVA.
The following conclusions can be drawn from the calculation and verification of the retrofit program.
After retaining the core, insulation reinforcement of the winding can reduce the insulation distance accordingly in order to increase the copper conductors as much as possible in the limited space; measures are taken to improve the short-circuit resistance.
The semi-hybrid insulation system, the use of natural ester insulating oil, the external increase of fans. transformer refurbishment temperature rise in line with the standard requirements, life can be comparable with the new transformer.
Transformer refurbishment cost is about 70% of the cost of a new transformer, which has a very high economic and social value. After verification and calculation, the expansion of 40MVA transformer to 50MVA is completely feasible.
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